THE MENSTRUAL CYCLE 



learned the importance of the periodic shutting off of the 

 spiral arteries. 



Coiled arteries of the type thus shown to be fundamentally 

 involved in menstruation in the monkey are also present in 

 the human uterus, but have never been found in non-menstru- 

 ating animals. Menstruation, then, is primarily an affair 

 of the coiled arteries, which control the blood supply of the 

 inner layer of the endometrium and by their closure cause 

 breakdown, tissue damage, and hemorrhage (Appendix II, 

 note 12). 



In view of the violent disruption that characterizes the 

 retrogressive phase of the cycle in women and in the other 

 menstruating primates, it is a matter of great theoretical 

 interest to know whether this stage in the non-menstruating 

 animals is actually as free from tissue breakdown as I have 

 rather summarily indicated. In other words, is menstruation 

 a totally peculiar affair, sharply different from what goes 

 on in mammals generally, or is it merely an exaggeration of 

 a degenerative process that is present but not extensive in 

 lower animals.? This question is being investigated, but the 

 answer cannot be given now. We need to know most of all 

 what goes on in the uterus at the end of the corpus luteum 

 phase in the New World monkeys (the capuchins, spider 

 monkeys, and howler monkeys), which in spite of their close 

 evolutionary relationship to the other primates do not 

 menstruate externally. Here, if anywhere, we may expect 

 to find transitional conditions that may help explain the 

 wherefore of menstruation. The evidence is not yet in, but 

 I may say that there are hints, apparent to the expert 

 microscopist, that even in the rabbit and other non-men- 

 struating mammals the retrogressive phase has an element 

 of acute damage in it. These signs are, however, slight indeed 

 and the statement holds true that in almost all mammals, 

 when the corpus luteum has done its work, and the uterus 

 is released from its phase of progestational proliferation, it 



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